Many types of processing equipment include a processing chamber operating at reduced pressure or in controlled ambient but require a sealable passageway into the processing chamber to allow a workpiece being processed or a large equipment used in the processing to be transferred at least occasionally between the processing chamber and another chamber or the exterior at a different pressure or ambient. As a result, the passageway needs be open for passage of the substrate or insertion of the equipment but closed during other phases of operation. That is, a large valve is required. Two additional requirements for the valve maybe resistance to high temperatures occurring within the adjacent processing chamber and that the action of the valve creates very few particles which would contaminate the processing chamber.
Two related valve types are often used if the passageway needs to be not only large but approximately circular. In a first type called a pendulum gate valve or swing valve, also simply referred to as a pendulum valve, a gate capable of sealing the passageway rotates about an axis offset from the passageway from a retracted position away from the passageway to an active or blocking position in the passageway at which it blocks the passage of large articles through the passageway. In a second type called a slider or shuttle valve, the gate moves laterally along a generally linear axis between the retracted and blocking positions. In either case, once the gate has reached the blocking position, it may block the passageway but it does not necessarily form a vacuum seal. To complete the sealing of the passageway, the gate needs to move generally along the axis of the passageway to engage a sealing surface surrounding the passageway. When the passageway needs to be unblocked, the gate needs to move away from the sealing surface before it is moved out of the passageway.
Although the invention is not so limited, one application of such valves involves the Czochralski growth of silicon ingots or boules in which a crucible filled or recharged with chunks or pellets of silicon is heated to above the melting point of silicon, approximately 1416° C., so that a melt of liquid silicon exists in the crucible. A small seed of silicon is lowered to the surface of the melt. If monocrystalline silicon is desired, the silicon seed should be monocrystalline and of the desired crystalline orientation. By careful control of temperatures near the silicon melting point, the liquid silicon freezes on the silicon seed and the seed grows into a larger piece of silicon of the same crystalline orientation as that of the seed. The growing silicon piece is slowly withdrawn and the process continues so that the width and axial length of the piece continue to increase. Again by careful control of temperatures and other growth parameters, the lateral size can be restrained to a desired diameter, for example, 200 mm or 300 mm desired for the present generation of silicon wafers. The desired product is a generally cylindrical ingot of monocrystalline silicon of the desired diameter and perhaps 2 m long. As the lower end of the ingot grows, the ingot is slowly drawn upwards into a pull chamber above the crucible. After the desired length of ingot is grown, the ingot is tapered down, separated from the melt, and withdrawn into the pull chamber. At least during the melting and growth of the silicon ingot, the crucible chamber should be maintained in an inactive ambient, for example, of argon, and preferably at a reduced pressure typically in the range of 10 to 50 Torr.
In batch Czochralski growth, the crucible is loaded with silicon chunks sufficient to complete the growth of one ingot. After the one ingot is grown, the crucible is typically cooled and then discarded and a new crucible is used for the next ingot. In batch Czochralski, it is typical to selectively isolate the pull chamber from the crucible chamber during the long heat up of the crucible and its charge and then to quickly lower the seed crystal from the pull chamber. Also, it is desirable to cool the ingot independently of the crucible. Conventionally, the valve between the crucible and pull chambers has been implemented as a flapper valve, which is effective but occupies valuable height in the pull chamber. It is desired to make the pull chamber as long as possible without requiring an excessively high ceiling in the factory.
In recharge Czochralski, after the growth of one ingot, the crucible is recharged with another batch of silicon chunks and the process is repeated for a further ingot. However, the recharge should be performed without significantly cooling the crucible and without disturbing the desired ambient of the crucible chamber. As a result, the new charge of silicon should be introduced through a load lock involving some kind of valved passageway.
In continuous Czochralski, only a limited amount of silicon is melted in the crucible but solid silicon is continuously or at least intermittently added to the crucible during the Czochralski drawing process and is immediately melted to augment the liquid. Additionally, multiple ingots are sequentially grown while the crucible remains filled with substantially the same amount of silicon melt. Clearly, the pull chamber must be valved to allow removal of the last grown ingot and the insertion of a new seed. Also, it is desired that the solid silicon charge contained in a hopper be pressurized to the pressure of the crucible chamber and the amount of the charge be less than the total charge required for the lifetime of the crucible. Therefore, some valving is required to isolate the crucible chamber from the hopper when it is being recharged even if this occurs during removal of a grown ingot. In a variant of continuous Czochralski, the silicon is pre-melted outside the crucible and flowed into the crucible to maintain a constant melt level in the crucible, but valving is still required to recharge the pre-melter with additional solid silicon.
Valves used in these Czochralski processes are subject to the two additional requirements of high temperature and low particulate production. Valves facing the interior of the crucible chamber operate with the gate facing a very hot crucible or crucible furnace but conventional seals such as elastomeric O-rings fail well below the temperature of the melted silicon. Even temporary exposure to hot parts may rapidly degrade the O-ring. Secondly, valves need to generate a minimum of particles which could fall into the crucible and contaminate the silicon ingot being produced. However, most valves involve some sort of sliding motion between two adjacent parts typically composed of stainless steel or other contaminating material.
Many pendulum valves accomplish the axial sealing motion by providing an axial movement to the shaft providing the rotary motion to the gate. However, axial movement of the rotary shaft is considered to generate excessive bending on the rotary shaft and large-area gate to provide the large sealing forces required to seal the gate and also to produce undesired particulates by the mechanical movements next to the passageway.
A valve should also be fail safe, for example, during a power failure or pump failure, that is, not uncontrollably change from its sealed to an unsealed condition or vice versa during the failure.